Add deep analysis of Linux thread pool implementation

[Copilot]

Documents the thread pool implementation in /src/prod/src/Common/Threadpool/ with identified issues, performance concerns, and alternative approaches.

Key Findings

Critical Issues

• nice() misuse - UnfairSemaphore.cpp:115 sets process-wide priority instead of thread-specific
• Silent thread creation failures - CreateWorkerThread() always returns true regardless of pthread_create result
• Clock skew vulnerability - Semaphore timeouts use CLOCK_REALTIME instead of CLOCK_MONOTONIC

Performance Bottlenecks

• Single SpinLock contention on work queue enqueue/dequeue path
• ARM64 YieldProcessor uses expensive sched_yield() syscall instead of yield instruction
• LifoSemaphore allocates shared_ptr<LifoEvent> per wait operation

Memory Ordering

• VOLATILE_MEMORY_BARRIER() is compiler-only barrier, insufficient for ARM weak ordering
• Potential data races in ThreadCounter on weakly-ordered architectures

Alternatives Proposed

• io_uring for async I/O (Linux 5.1+)
• epoll + eventfd for worker wakeup
• Work-stealing queues for reduced contention
• C++20 std::counting_semaphore / std::jthread

Changes

• Added docs/ThreadPoolLinuxAnalysis.md with full analysis, code references, and prioritized recommendations

Original prompt

Deep aanlysis of the thread pool implementation for Linux, probable issues and alternate solutions.

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